1. Field of the Invention
This invention relates to a servo system which controls the rotational speed of a rotational body such as a rotating motor or the moving speed of a linear moving body such as a linear motor so that the rotational or moving speed becomes a desired speed.
2. Description of the Prior Art
A servo system for controlling the rotational speed of a rotational body has conventionally a speed detector which generates an AC signal whose voltage or frequency represents the rotational speed of the rotational body. This system is called the tachogenerator servo. There are two kinds of speed detection method--voltage detection method and frequency or period detection method.
There are some examples of the voltage detection method. For example, the amplitude of the output AC signal of the speed detector which possesses a generating coil changes in corresponding with the moving speeds. When this AC signal reached a predetermined voltage, a switching transistor is operated to charge a capacitor. When the switching transistor is in OFF condition, the charging of capacitor is carried out by a constant resistor. With this constraction, an error voltage depending on the moving speed is obtained (for example, disclosed in Japanese Published Patent No. 58-6392). Another method obtains the error voltage by rectifying the output AC signal of the speed detector. Still another method directly uses the generated AC voltage of the speed detector (for example, disclosed in U.S. Pat. No. 2,905,876). In this example a chopper is used to use the controlled motor as the speed generator during its unenergized period.
However, in these systems the generated voltage of the speed detector used as a speed information changes easily due to the change of ambient temperature and long use. Thus, they can only be used as low grade servo systems.
The frequency or period detection method uses the frequency or the repeated period of the output AC signal of the speed detector as a speed information. Digital servo systems using this method (for example disclosed in Japanese Published Patent No. 53-19745 or U.S. Pat. No. 3,836,756) have the advantage of very high stability. The frequency or period detection method produces a speed error from specified edges of the well-amplified (to have a square waveform) output AC signal of the speed detector. For example, a representative period detection method counts clock pulses in a period from one leading edge of the amplified output signal of the speed detector to the next leading edge, the count value corresponding to the moving speed of the moving body. From this count value a pulse width modulation signal is produced (in the case of the chopper type driving method) or the count value is converted to an analog voltage thereby to obtain the speed error output.
Therefore, if we try to realize a higher resolution control, it is necessary to increase the number of edges. For example, when the AC generator which generates one cycle of AC signal in one revolution of the motor is used in the edge detection method, it is impossible to obtain two or more speed error signals during one revolution of the motor. By using both the leading edge and the trailing edge of the square wave signals, at most two speed error signals could be obtained.
Other conventional methods are the method of using PLL (Phase Locked Loop) to multiply the frequency of the output AC signal of the speed detector (disclosed in U.S. Pat. No. 4,114,075) and the method of obtaining the speed detection signal from two kinds of AC signals which differ in phase by .pi./2, the speed detection signal thus having four times frequency (for example, disclosed in Japanese Published Patent No. 58-6165). However, since the speed information obtained from the multiplied signal is dependent on the speed information of the original signal, no effect is obtained in the purpose of improving resolution of control. In the latter method, the construction of the speed detector is relatively complicated and, compared with the previously explained method of using both the leading and trailing edges of the output AC signal of the speed detector, the resolution is improved by only two times higher.
Because of the above, so far efforts have been made for increasing the output frequency of the speed detector. However, even though the output frequency of the speed detector is increased, large effect will not be expected. Also, the construction of the speed detector becomes complicated. (For example, by radiating laser beam onto the speed detection track which is formed by photomask etching, and detecting its reflective light, frequency of speed detector greatly increases, but the system construction becomes much complicated. Moreover, high precision manufacturing technique is required for such systems.